Compositions of yellow emitting zinc sulfide electroluminescent phosphors are disclosed in the following U.S. Pat. Nos.: 2,743,238; 2,772,242; 3,076,767 and 3,775,173.
U.S. Pat. No. 2,743,238 teaches that a zinc sulfide electroluminescent phosphor can be activated with copper in the range of 0.05% to 1% by weight and manganese in the range of from 0.1% to 10% by weight. The patent also teaches that the brightness of the phosphor is improved by mixing the zinc sulfide with about one third of its weight with zinc oxide, which is washed out of the phosphor with an acetic acid wash subsequent to the firing of the materials. In an alternate process a portion of the zinc sulfide is converted to zinc oxide by controlled oxidation during the firing process.
U.S. Pat. No. 2,772,242 teaches that the addition of a small amount of lead further improves the phosphor and the use of a wash to dissolve free zinc oxide after firing also increases the luminescence. This patent teaches against the use of zinc oxide because of excessive conductivity. From about 0.0001 to about 0.03 gram-atom of manganese per mole of ZnS, corresponding to about 0.01% to about 14.5% by weight of manganese, is the disclosed level of manganese.
U.S. Pat. No. 3,076,767 teaches that when a phosphor is prepared by first forming a zinc sulfide material containing copper and chloride, and thereafter, that material is etched with hydrogen peroxide, then manganese carbonate and copper sulfate are added and the material is refired, the brightness of the resulting phosphor is better than the unetched phosphor. The level of retained manganese is not disclosed, however, it is stated that after drying, 4% manganese (as manganese carbonate) is added and the material is fired at 800.degree. C. The phosphor has an efficiency of 4.4 lumens per watt.
U.S. Pat. No. 3,775,173 teaches that an electroluminescent zinc sulfide phosphor activated with manganese is improved by immersing the activated material in and aqueous solution of a copper salt. The level of retained manganese is disclosed as being from 0.1% to 0.5 % by weight. Manganese chloride is used as the manganese source.
Commerical electroluminescent zinc sulfide phosphors are activated with about 1.3% by weight of manganese, about 0.03% by weight of chloride and about 0.05% by weight of copper and have and a lumens efficiency of about 5.2 lumens per watt when measured at 115/400 Hz using conventional electroluminescent lamps.